This invention relates to copolymers of a rigid-rod polymer unit such as poly(p-phenylenebenzobisoxazole) with a flexible backbone unit based on the hexafluoroisopropylidene group. Also described are copolymers containing the rigid-rod polymer unit poly(2,5-dihydroxy-1,4-phenylenebenzobisoxazole) with a flexible copolymer unit containing the hexafluoro-isopropylidene group in its backbone.
The rigid-rod copolymers described herein will also extend to poly(p-phenylene-benzobisthiazole) (PBZT) and poly(p-phenylenebenzobisimidazole) (PBI) copolymer compositions containing the flexibilizing hexafluoroisopropylidene units as well as to the dihydroxy variants of the above.
Aromatic heterocyclic rigid-rod polymers, such as PBO are known to have unique mechanical properties and exceptional thermal and thermo-oxidative stabilities. The commercial version of the heat-treated PBO fiber, known as ZYLON™, is reported to exhibit measured tensile strength of 5.8 GPa and a tensile modulus of 270 GPa while it has an elongation at break as low as 2.5%. Nago, S., et al., JP 2003251704. The onset of decomposition temperature in air is ˜650° C. and its thermal stability in an inert atmosphere clearly exceeds 700° C. Because of its outstanding attributes, ZYLON™ is utilized as a high performance material in a variety of applications such as protective clothing, sports and aerospace, to mention just a few. Specific examples include flame/heat-resistant fabrics, ballistic vests, balloons, satellites, sailcloth, yacht ropes, golf clubs and reinforcement for cement and for belts and tires. However, some instances of in-service failure/degradation of the ZYLON™ fibers have stimulated recent studies on the effects of environmental conditions such as moisture, heat, as well as UV-radiation on PBO fiber properties. Hydrolytic degradation of PBO due to exposure to moisture, especially in presence of residual acid, has been investigated. PBO fiber degradation mechanism due to exposure to UV radiation has also been a subject of scrutiny.
In recent years, PBO has also been evaluated in insulator substrates as fibers bonded to epoxy resins and in non-woven fabric-based battery separators as composite fibers incorporating semi-aromatic polyamide fibers. See Japan Tokkyo Koho 2006/022433 and 2005/022836. PBO has also been utilized in composite solid polymer electrolyte membranes as a porous polymer substrate interpenetrated with an ion-conducting material for electrochemical applications such as fuel cells. See U.S. Pat. No. 7,052,793. Fiber-reinforced thermoplastic composites with high tenacity have also been fabricated by impregnating ZYLON™ fibers with an ethylene-vinyl alcohol copolymer. Besides the formation of blends and composites involving PBO, some instances of copolymerization involving PBO have also been reported. Conjugated random copolymers of rigid-rod PBO with extended-rod poly(2,5-benzoxazole) (ABPBO) and 2,5-thienyl benzobisoxazole (PBOT) units have been synthesized for study of their opto-electronic properties.
Dihydroxy-pendant variations of rigid-rod PBO (DiOH-PBO); PBZT (poly(p-phenylenebenzobisthiazole)) (DiOH-PBZT) and the corresponding benzimidazole DiOH-PBI have been reported. See U.S. Pat. Nos. 5,041,522 and 5,016,940.
Thermoplastic polybenzoxazoles (6F-PBOs) derived from bis(3-amino-4-hydroxyphenyl)hexafluoropropane, incorporating wholly flexible polymer backbone are disclosed in the literature. They have been evaluated in a number of applications ranging from photo-resist compositions for electronic patterning to interlayer insulating dielectrics for micro-electronic applications and as flexible plastic substrates for liquid crystal displays. See WO 2007/148384 and U.S. Pat. No. 6,057,417. Thermoplastic 6F-copolymer compositions incorporating various aromatic as well as cycloaliphatic structural units have been reported by Hasegawa, T., Japan Kokai Tokkyo Koho 2006/143943. 6F-benzoxazole-imide copolymers derived from the thermal cyclodehydration of the poly(amic acid-hydroxyamide) precursors are also known. Hsu, S. L-C.; Luo, G-W.; Chen, H-T.; Chang, S-W., J. Poly. Sci., Poly. Chem., 2005, 43, 6020.